In wireless network systems such as Wideband Code Division Multiple Access (WCDMA) and Long Term Evolution (LTE), discontinuous reception (DRX) mode can be used to reduce power consumptions in user equipments (UE). FIG. 1 illustrates a DRX cycle or DRX period which includes on-duration and DRX opportunity periods. During the on-duration period, the UE turns on its receivers to listen for scheduling information on downlink control channels from the network. In LTE for example, scheduling information is transmitted by the eNodeBs on the Physical Downlink Control Channel (PDCCH). During the DRX opportunity period, the UE may turn off its receivers, i.e., go to sleep, to reduce battery consumption. The DRX mode is important to increase the standby times of mobile devices such as small handsets.
Also in wireless systems, the mobility of a UE in terms of handover from one cell to another or a cell reselection can be either network controlled or UE controlled. In the network controlled mobility mode, the nodes on the network's infrastructure side such as the radio base station (RBS, eNodeB) and the radio network controller (RNC) are in charge of moving the UE from one cell to another. The network controlled mobility mode is typically UE assisted in which the UE measures the signal strengths of neighboring cells, and provides measurement reports to the network. Based on these reports, the network decides whether and when a handover should be executed. Handovers are typically issued by messages from the network to the UE, where the UE is commanded to perform the handover to a specific cell.
In the UE controlled mobility mode, the UE is allowed to autonomously perform cell reselection, i.e. the UE is free to select a new cell based on measurements of signals from multiple cells and application of some selection criteria and thresholds. The criteria and thresholds can be provided by the network. Typically, the UE reports to the new cell or to the cell area when the cell reselection is completed.
Wireless systems can deploy both mobility solutions. In LTE for example, radio resource control (RRC) protocol is modeled with two states—RRC_IDLE and RRC_CONNECTED. One difference between these two states is the applied mobility solution. In the RRC_IDLE state, the UE controlled mobility is implemented in which the UE performs the measurements and the cell reselection.
In the RRC_CONNECTED state, the network controlled mobility is implemented in which the network is in control of when the handover and cell reselection for the UE occurs. In this state, the UE's location is known to the network to at least on a cell granularity level, and explicit RRC signalling is involved when the UE moves from one cell to another cell.
In the RRC_IDLE state on the other hand, the UE's location may not be known exactly. When the network needs to reach the UE, paging that spans a larger tracking area consisting of multiple cells may be required. Thus, the UE only needs to report its cell change when it leaves its current tracking area which can span multiple cells. This reduces both signaling and UE battery consumption since the UE can move between multiple cells without being engaged in any signaling.
LTE supports the DRX mode in both the RRC_IDLE and RRC_CONNECTED states. In the RRC_IDLE state, the sleep periods that the UE can apply are primarily constrained by the paging period, i.e., the periodicity at which the UE needs to read the downlink channels from the network to find out whether there are any paging messages directed to the UE. Typical paging periods range from hundreds of milliseconds to several seconds. Between these paging opportunities, the UE can be asleep.
In the RRC_CONNECTED state, depending on the UE's level of activity, the UE can successively go down into deeper sleep modes. The activity of the UE refers to circumstances in which the UE is scheduled to receive messages from or transmit messages to the network. The UE is “active” if it finds itself scheduled for uplink and/or downlink communication. The UE is not considered to be active if it is only periodically waking up to read paging or system information.
Referring back to FIG. 1, the duration between the on-duration periods offers an opportunity for the UE to turn off its receivers. In LTE, two configurable DRX cycles are supported—a short DRX cycle and a long DRX cycle, c.f. TS 36.321. The UE that is inactive may stepwise increase the lengths of its DRX opportunity periods and correspondingly increase its sleep periods to improve battery preservation. A range of configurable DRX cycles in the RRC_CONNECTED state can be comparable to the paging cycles. Thus, it is possible to configure very efficient DRX also in the RRC_CONNECTED state such that a UE in the RRC_CONNECTED state can have similar standby times as a UE in the RRC_IDLE state.
In the RRC_CONNECTED state as noted above, the network controlled mobility is implemented in which the UE measurements are used to assist in the handover decision. However, a UE in a long DRX cycle cannot offer equally precise neighbor cell measurements. To conduct the cell measurements, the UE's receivers must be turned on. To provide high measurement accuracies, the UE must turn on its receivers more frequently, and frequent measurements hinder the UE from utilizing the DRX opportunities to save power.
Wireless systems typically also include functionalities to monitor the quality of the present radio link between the UE and the network, and a radio link failure (RLF) is detected when certain criteria are fulfilled. When the RLF occurs, appropriate actions are taken to recover or reestablish connection between the UE and the network.
Similar to the aforementioned mobility measurements, the radio link quality measurements also require the receivers be turned on. Thus, the RLF detection is dependent on the sleep-periods applied to the receivers. To facilitate long DRX opportunities, 3GPP requirements specify less stringent radio link quality monitoring when the UE is in a sleep mode, c.f. TS 36.133, clause 7.6.
In the RRC_CONNECTED state, the network controlled mobility applies until the UE declares an RLF, after which, the UE is allowed to select a better cell to recover the connection. Unfortunately, the declaration of the RLF and recovery therefrom can take a considerable amount of time, during which the UE may lack means to transmit or receive any data.
It is seen that a combination of long DRX cycles and UE assisted, network controlled mobility pose a tough challenge. For example, an inactive UE with a long DRX moving towards a cell border may not provide sufficiently accurate measurements to the network or may even fail to provide reports at all due to delay, thereby resulting in a failed handover or a radio link failure. Providing accurate measurements on the other hand would waste the power-saving opportunities offered by the DRX periods.
Since both the mobility measurement accuracies and the RLF criteria are functions of the DRX periods, the UE may remain without means to transmit or receive data during a non-negligible amount of time. The network will be unaware of this status of the UE, as the UE may not have been able to report any measurements to the network before the problem arises. In the RRC_CONNECTED state, the UE is allowed to communicate with the serving cell only, i.e, the cell the UE is currently connected with. However, without knowledge of the UE status, the network is not able to move the UE to a better cell.
Another problem is that when the UE is at a cell border, measurement reports may be triggered repeatedly. Whenever a measurement report is triggered, the UE leaves the DRX to transmit the report. If this situation prevails, the UE will not be able to remain in a battery saving mode, and the UE stand-by time in the RRC_CONNECTED state will be reduced significantly.